Tin immersion plating composition and process for using the same



United States Patent '0 TIN 1M1VIERSION PLATING COMPOSITION AND PROCESSFOR USING THE SAME Myron Ceresa, Penn Township, Allegheny County, and

John R. vDrobne, Munhall, Pa., assignors to Westinghouse ElectricCorporation, East Pittsburgh, Pa., a corporation of Pennsylvania NoDrawing. Application September 21, 1956 Serial No. 611,365

3 Claims. (Cl. 106-1) The present invention relates to plating tin onsurfaces of copper and copper base alloys by chemical displacement. Morespecifically, the invention relates to the deposition of coatings ofmetallic tin on such cupreous surfaces by an immersion plating processwithout the use of an electric current.

In the past, coatings of tin have been deposited on articles byimmersion plating techniques employing aqueous solutions containingstrong liquid mineral acids such as hydrochloric acid and sulphuricacid. The use of such strong mineral acids in immersion plating aspreviously practiced, generally resulted in a pronounced attack on thecopper or copper base alloys on which the tin was deposited. Moreover,the strong mineral acids appear to dissolve a portion of the tin coatingdeposited on the article being plated.

The object of the present invention is to provide a .nonmineral acidcontaining composition, adapted for mixing with water, from whichaqueous solution tin may be immersion plated on to copper and copperbase alloys without disadvantageous results obtained in prior artimmersion plating techniques.

Another object of this invention is to provide a solution adapted forthe immersion plating of tin on cupreous articles comprising a watersolution of thiourea or derivatives thereof, at least one water-solublestannous salt, and at least one normally solid, water-soluble,non-mineral acid.

Still another object of the present invention is to provide a method ofdepositing a coating of tin on cupreous metal by contacting said metalwith an immersion plating solution for a period of time sufiicient todeposit a tin coating of desired thickness thereon, the solutioncomprising water, thiourea, or derivatives thereof, at least onewater-soluble stannous salt, and at least one normally solid,water-soluble, non-mineral acid.

Other and further objects of this invention will, in part, be obviousand will, in part, appear hereinafter.

In accordance with the present invention and in the attainment of theforegoing objects, there is provided a composition adapted for mixingwith water at time of use to provide an aqueous solution from which tinmay be plated on to articles of copper and copper base alloys byimmersing such articles in said solution, without the use of an electriccurrent.

The composition, which is to be dissolved in water to form the immersionplating solution of this invention, is a mixture of dry componentscomprising (a) from 7.7% to 98.5% by weight of thiourea and certainwater-soluble derivatives thereof, (b) from 0.2% to 64.5% by weight ofat least one Water soluble stannous salt, and (c) from 0.9% to 90.5% byweight of at least one normally solid, water-soluble, non-mineral acid.

Bright, firmly adhering coatings of tin may be deposited on cupreousmetal articles when the same are immersed in aqueous solutions of theabove described mixture when said mixture is dissolved in water inproportions amounting to from about 2 to 35 ounces of said mixture pergallon of water.

It is preferred to use thiourea in the compositions of this invention.Satisfactory results are also obtainable using derivatives of thioureaor, more particularly, watersoluble compounds of the general formula iti! i wherein R represents at least one radical selected from the groupconsisting of hydrogen, an acetyl radical, and aliphatic hydrocarbon andaliphatic monohydroxy hydrocarbon radicals containing from 1 to 2 carbonatoms per radical. Examples of aliphatic hydrocarbon radicals includemethyl, ethyl, methylene, ethylene, and the like and examples ofmonohydroxy hydrocarbon radicals include methylol, ethylol, and thelike.

Satisfactory tin coatings may be applied on surfaces of cupreousarticles using the aqueous immersion plating compositions of thisinvention by contacting said surfaces with the solution for a period oftime sufficient to deposit a tin coating thereon of desired thickness.Immersion periods varying from 1 to 15 minutes have been found toprovide bright firmly adhering tin deposits on cupreous surfaces. Thethickness of the tin coating deposited on the cupreous surface treatedwith the plating solution of this invention depends on the particularconcentration of solid components dissolved in the solution, the lengthof time the cupreous surface is in contact with the solution, and thetemperature of the solution. The solution provides satisfactory platingat any temperature, i.e., from room temperature up to about the boilingpoint of the solution.

After the cupreous article has been immersed in the solution for asufficient period of time and at a desired temperature to produce a tindeposit of desired thickness, the article .is removed from the solutionand may be washed or rinsed with water. The tin deposit formed isbright, smooth, firmly adherent to the under lying cupreous surface, andfree from pin holes or other defects.

Articles having a cupreous surface, which may be coated by immersing thesame in the aqueous compositions of this invention, include articles ofcopper or cop per base alloys such as, for example, red brass, andbronzes, as well as members which have a surface layer of copper or acopper base alloy thereon, such as, for example, copper or brass platedsteel-cored articles.

Tin is provided in the aqueous plating solutions of this invention inthe form of at least one water-soluble stannous salt. Stannous chlorideis particularly satisfactory and is a preferred source of tin informulating the compositions of this invention. Other water-solublestannous salts, such as, for example, stannous bromide, stannous iodide,and stannous sulfate, also may be employed with satisfactory resultsbeing obtainable. It is also possible to employ other tin salts such as,for example, sodium st-annite, which contains tin in the stannouscondition. These tin salts may be used singly or in mixtures of two ormore.

The acids employed in formulating the aqueous immersion platingcompositions of this invention are normally solid, water-soluble,non-mineral or organic acids. Examples of suitable acids include citricacid, tartaric acid, sulfamic acid, malic acid, maleic acid andlevulinic acid. 'l'lhese acids may be used singly or in mixtures of twoor more.

A thiourea also is included in the compositions of this invention.Suitable thioureas are those selected from that class of compoundscontaining a bivalent sulfur atom both bonds of which are connecteddirectly to a carbon atom and having at least one nitrogen atomconnected directly to the carbon atom. Thioureas having the nucleus areparticularly suitable. Thiourea, CS(NH is preferred since it is perhapsthe most readily available compound of this class and its use affordsexcellent plating results.

In order to indicate more fully the advantages and capabilities of thepresent invention, the following specific examples are set forth. Thepercentages given are by weight unless otherwise indicated.

Example I Example I] An aqueous plating composition is prepared bydissolving a mixture comprising 58% thiourea, 8% stannous bromide and34% or" tartaric acid in water, the solution comprising 10 ounces of themixture of solids per gallon of deionized water. Excellent tin depositsresult upon immersing cupreous articles in this plating composition.

Example III A tin immersion plating solution is prepared by dissolving asolid mixture comprising 80% of thiourea, 4% of stannous iodide and 16%of sulfamic acid in deionized water. Bright, firmly adherent tincoatings are deposited on cupreous metal articles when such articles areimmersed in an aqueous solution containing 12 ounces of this mixturedissolved in one gallon of water. Comparable results are obtainable bysubstituting dimethyl thiourea for the thiourea.

Example IV A continuous coating of tin is deposited on a copper articleby immersing such article in an aqueous solution containing 14 ouncesper gallon of a solid mixture including 67% thiourea, 7% stannous iodideand 26% malic acid.

Example V An aqueous tin immersion plating solution is prepared bydissolving 14 ounces of the following mixture of solids in one gallon ofWater.

Percent Thiourea 85 Stannous sulfate r- 3 Maleic acid 12 A smoothcontinuous coating of tin is deposited on copper articles when sucharticles are immersed in this composition at a temperature of 40 C. for5 minutes. Equally satisfactory results are obtainable by substitutingmonomethylol thiourea for the thiourea and heating the aqueous solutionto 60 C.

Example VI An aqueous tin immersion plating bath is prepared bydissolving 14 ounces of a solid mixture containing 40% thiourea, 15%sodium stannite and 45% levulinic acid in a gallon of water. A coppercoated steel article is immersed in this bath for 15 minutes. It then isremoved and rinsed with Water. A firmly adherent coating of tin is foundto have been deposited on the copper surfaces.

The aqueous tin immersion plating compositions of this invention aremore stable and capable of use for longer periods of time than theimmersion plating solutions previously known in the art. It has beendetermined that there is an optimum concentration of the three essentialsolid ingredients of the compositions of this invention which permitsplating the greatest number of cupreous articles Without the need ofadding additional quantities of the three ingredients to compensate fortheir consumption during plating. Experience has shown that this optimumcomposition contains the three essential ingredients in substantiallythe following proportions.

Percent Thiourea 55 Water-soluble stannous salt 6 Non-mineral acid 39The most satisfactory plating results are obtained when an aqueouscomposition is prepared by dissolving 12 ounces of the above mixture inone gallon of water, though from 10 to 20 ounces per gallon giveexcellent results.

It has been determined that cupreous articles having a greasy orotherwise dirty surface may be provided with a more uniform, bright anddense tin deposit if there is included in the immersion baths of thisinvention from 0.01% to 10% by weight of a wetting agent such as analkyl aryl sulfonate.

While the present invention has been described with particular referenceto preferred embodiments thereof, it will be understood, of course, thatcertain changes, substitutions, modifications, and the like, may be madetherein without departing from its true scope.

We claim as our invention:

1. A solid composition adapted for mixing with water to provide asolution in which tin may be immersion plated onto copper and copperbase alloys consisting essentially of from 7.7% to 98.5% by weight of awater soluble compound of the general formula wherein R represents atleast one radical selected from the group consisting of hydrogen, anacetyl radical, and ali phatic hydrocarbon and aliphatic monohydroxyhydrocarbon radicals containing from 1 to 2 carbon atoms per radical,from 0.2% to 64.5% by weight of at least one water-soluble stannous saltselected from the group consisting of stannous chloride, stannousbromide, stannous iodide, and stannous sulfate, and from 0.9% to 90.5%by weight of at least one normally solid water-soluble non-mineral acidselected from the group consisting of citric acid, tartaric acid,sulfamic acid, malic acid, maleic acid, and levulinic acid, saidcomposition being adapted for mixing with water to provide a tinimmersion plating solution in which the components cooperate with oneanother to provide improved plating.

2. A solution adapted for immersion plating tin therefrom on copper andcopper base alloys consisting essentially of a water solution of amixture of from 7.7% to 98.5% by Weight of thiourea, from 0.2% to 64.5%by weight of stannous chloride, and from 0.9% to 90.5% by weight of atleast one normally solid water-soluble non-mineral acid selected fromthe group consisting of citric acid, tartaric acid, sulfamic acid, malicacid, maleic acid, and levulinic acid, said solution consistingessentially of from 2 to 30 ounces of said mixture per each gallon ofWater.

3. A method of depositing a coating of tin on a surface of a cupreousarticle which comprises contacting said article with an aqueousimmersion solution for a period of time suificient to deposit a tincoating of desired thickness thereon, and thereafter removing saidarticle from levulinic acid, said solution consisting essentially ofcontact with said aqueous solution, said aqueous solution 2 to 30 ouncesof said mixture per each gallon of water. consisting essentially of amixture of from 7.7% to 98.5%

by weight of thiourea, from 0.2% to 64.5% by weight References Cited mme of flns patent of stannous chloride, and from 0.9% to 90.5% by weight5 UNITED STATES PATENTS of at least one normally solid water-solublenon-mineral 1,903,860 Gockel Apr. 18, 1933 acid selected from the groupconsisting of citric acid, 2,369,620 Sullivan et a1. Feb. 13, 1945tartaric acid, sulfamic acid, malic acid, maleic acid, and 2,766,138Talmey Oct. 9, 1956

1. A SOLID COMPOSITION ADAPTED FOR MIXING WITH WATER TO PROVIDE ASOLUTION IN WHICH TIN MAY BE IMMERSION PLATED ONTO COPPER AND COPPERBASE ALLOYS CONSISTING ESSENTIALLY OF FROM 7.7% TO 98.5% BY WEIGHT OF AWATER SOLUBLE COMPOUND OF THE GENERAL FORMULA